Generally speaking, a compound lens, consisting of any number of lenses in any of various combinations (e.g., seven lenses in six groups or eleven lenses in ten groups), is often used in an imaging optical system such as a camera. According to a paraxial theory, such a compound lens is equivalent to a single convex lens. However, such approximation is accurate enough only within a relatively narrow range near the optical axis. Accordingly, in actually handling a rather big image, an aberration associated with an angle of view is a problem. Also, in treating multiple wavelengths as in a color image, a chromatic aberration is a problem.
To reduce such aberrations, various combinations of refractive indices and optical surface shapes have been proposed for use in lenses to make up a compound lens. However, to minimize the aberration associated with the angle of view and the chromatic aberration over the entire area of a photodetector, the compound lens needs to consist of a lot of lenses, which increases the overall cost of the required members, the assembling and adjustment costs, and the size of the optical instrument, too.
In order to reduce the overall size of a lens system, it is effective to increase the refractive powers of respective lenses. However, the aberrations resulting from the angle of view and the difference in wavelength would rather increase by doing just that. Nevertheless, if the number of lenses to make up the compound lens were increased to avoid that situation, then the optical instrument would get too much complicated or bulky.
As can be seen, reduction of the aberrations and simplification of the optical system are contradictory purposes, which are hard to achieve at the same time and are still key issues in optics design of today.
Also, in a zoom lens optical system, for example, the aberrations must be reduced with the magnification of imaging changed, thus making it even more difficult to simplify and downsize the optical system.
Furthermore, not just the lens system but also a light quantity control mechanism such as a diaphragm and a shutter increase the size of an optical instrument. This is particularly remarkable in an optical system with a compound lens consisting of a lot of lenses such as a wide-angle lens or a zoom lens. However, depending on the angle of incidence of a bundle of rays entering a lens, the bundle of rays may pass just a portion of each lens, thus making it difficult to design and arrange the optical elements so as to adjust the light quantity uniformly over the entire area of the photodetector. For example, the diaphragm must be arranged at a position where the illumination can be changed at the same ratio as the center portion over the entire area of the photodetector. This condition imposes a constraint in designing an optical system.
The presence of such a light quantity control mechanism and the constraints on its arrangement constitute another obstacle to simplifying and downsizing the optical system.
Meanwhile, not only conventional passive optical elements with fixed optical surfaces such as a glass lens or a prism but also active optical elements with deformable optical surfaces have been developed recently.
Examples of those active optical elements include an optical element that uses a lens with an encapsulated transparent liquid as a variable-focus lens by getting the lens driven by a piezoelectric element (see, for example, Japanese Laid-Open Publication No. 2001-257932, which will be referred to herein as “Document No. 1”). Document No. 1 discloses a configuration in which multiple images are sampled with the focal point of this variable-focus lens switched at high speed and an image that is in best focus with the object is extracted by an image processing technique, thereby obtaining an all in focus image. This Patent Document No. 1 also discloses a configuration for correcting the shift of an image (e.g., variation in magnification and distortion of the image) resulting from a variation in focal length.
Also, in another proposed configuration, a deformable mirror called “Digital Micromirror Device” (DMD, a product name) is provided on the optical path of an imaging optical system and the number of ON-state mirrors or the time of their ON-state periods is controlled, thereby making up an optical diaphragm device (see, for example, Japanese Laid-Open Publication No. 11-231373, which will be referred to herein as “Document No. 2”). This Document No. 2 discloses a configuration for obtaining an arbitrary aperture shape by controlling the mirror driving pattern.
However, the conventional configurations described above have the following drawbacks.
Firstly, only a few types of aberrations can be corrected and reduction of the aberrations over the entire area of the photodetector and simplification of the optical system are hard to realize at the same time. Document No. 1 does disclose a configuration that can be used effectively to correct the focal point shifting and image distortion but does not disclose any configuration for reducing aberrations of different modes such as coma aberration and astigmatism. For example, the aberration associated with the angle of view is mostly a coma aberration or astigmatism, which is hard to correct effectively by the configuration disclosed in Document No. 1. Likewise, a chromatic aberration due to a wavelength difference or an aberration caused by a change of the magnification of imaging includes a lot of components that can never be corrected just by changing the focal length, and is also hard to correct effectively according to the configuration disclosed in Document No. 1.
Accordingly, even when the configuration of Document No. 1 is adopted, a compound lens consisting of almost the same number of lenses is also needed to reduce the angle-of-view aberration and chromatic aberration over the entire area of the photodetector. Consequently, reduction of the aberrations and simplification of the optical system are still difficult to achieve at a time to virtually the same degree. Document No. 2 does not provide any particular configuration for overcoming this problem, either.
Secondly, the aberration correcting mechanism and light quantity control mechanism are both required, thus making it difficult to simplify the optical system. In the configuration of Document No. 2, an imaging optical system with a compound lens consisting of a lot of lenses is provided as the aberration correcting mechanism and a DMD and its driver circuit are provided as the light quantity control mechanism. These are separately provided as independent mechanisms for fulfilling distinct purposes. And Document No. 2 discloses no configuration that can function as both of these two mechanisms at the same time. Thus, the presence of these two mechanisms increases the number of members required, the complexity of the assembling and adjustment processes, and the overall cost and size of the entire optical system. Document No. 1 does not provide any particular configuration for overcoming this problem, either.
Thirdly, it is difficult to reduce the size of the optical system due to the constraint on the arrangement of the light quantity control mechanism. In the configuration of Document No. 2, the quantity of the incoming light is controlled by getting the incoming light deviated by the DMD out of the range of the photodetector. However, to deviate the incoming light totally outside of the photodetector, at least a certain distance must be provided between the photodetector and the DMD. This distance heavily depends on the angle of deviation caused by the DMD and the size of the photodetector, which makes it difficult to downsize the optical system.
Furthermore, in the configuration of Document No. 2, the incoming light is once converged toward a point, where the DMD is provided as a diaphragm. Thus, the overall optical system has an increased size. In Document No. 2, the constraint on the arrangement of the diaphragm to control the light quantity uniformly over the entire area of the photodetector interferes with the desired downsizing. In this respect, Document No. 2 discloses any particularly effective means for improvement. Document No. 2 discloses a configuration for obtaining an arbitrary aperture shape by controlling the mirror driving pattern. However, it is impossible to independently control the light quantities for respective areas of the photodetector with only one aperture shape. Thus, the non-uniformity of the light quantity control cannot be eliminated essentially. Document No. 1 does not provide any particular configuration for overcoming this problem, either.
It should be noted that Japanese National Phase Publication No. 2002-525685 discloses a technique of getting light rays, which have come from multiple different portions of an object, sequentially imaged on a single photosensitive region one after another but still cannot overcome the problems described above.
In order to overcome the problems described above, an object of the present invention is to provide an optical detection system, which can reduce the aberrations over the entire area of the photodetector, can control the light quantity appropriately, and can simplify the optical system at the same time.